1. Field of the Invention
The present invention relates to a dual element-type pyroelectric infrared detector and, more particularly, to an improved configuration of the sensing elements to increase the accuracy of detection and the direction of movement of objects sensed.
2. Description of Related Art
Infrared detectors have been utilized to determine the entrance and presence of persons through the use of a pair of sensing elements. Pyroelectric infrared detectors of this type have been used as burglar alarms or intruder alarms, and are usually arranged in a room to provide a relatively long monitoring distance and the capability of specifying the movement of a person into or out of the room from a specific direction. Reference can be made to FIG. 7(A) to disclose a plan view of a pyroelectric infrared detector of a conventional configuration. The light-receiving elements 32 and 33 have the same rectangular configuration and are mounted on a substrate member 31 for receiving infrared radiation. The specific light-receiving areas are arranged to be symmetric about a point and relatively close to each other to form a dual element. FIG. 7(B) shows a schematic electric circuit for processing signals from such a dual element detector. A high megohm load resistance 34 is used to regulate the sensitivity and time constant of the circuit. An impedance-converting J-FET 35 is connected to, respectively, a supply voltage pin D and an output pin S. A ground or earth pin E is also disclosed in the circuit.
An advantage to this form of pyroelectric infrared detector is that no difference in output signal can be produced between both light-receiving electrodes 32 and 33. That is, the outputs from both light-receiving electrodes 32 and 33 and the circuit of FIG. 7(B) are connected to cancel each other when subject to ambient conditions. Thus, random noises can be eliminated because each of the light-receiving areas of these electrodes are equal to each other and will respond in a common manner to noises such as sunlight, internal lighting, vibration, and mechanical shock.
A problem exists, however, in the response characteristics of this type of pyroelectric infrared detector in that its sensitivity is dependent upon a particular movement or direction of the sensing target. These targets are disclosed, respectively, as vector symbols A, B, C, and D in FIG. 7(A).
Referring to FIG. 8, the output signal response associated with the movement of the person or target object from left to right (A direction), right to left (B direction), rear to front (C direction), and front to rear (D direction) is disclosed. A sensitivity orientation characteristic diagram corresponding to these measurements is plotted in FIG. 9. While a moving direction can be found under certain circumstances in FIG. 8, it can also be seen from FIG. 8 that a pyroelectric infrared detector of that configuration is hardly responsive to a moving body or person approaching in a corresponding angle of 90 degrees or 270 degrees. This is defined as the detection dead angle for this detector.
Referring to FIG. 9, the sensitivity orientation characteristic was obtained by applying infrared rays through a slit corresponding to the pyroelectric infrared detector at a pitch of 45 degrees and measuring an output voltage at that time, followed by converting the output voltage into a polar coordinate. Accordingly, with a pyroelectric detector of this configuration, it is not possible to accurately determine a moving body in certain predetermined sensing areas or to detect the moving direction of the moving body.
In order to address such requirements, a dual twin-type pyroelectric detector has been proposed, as shown in FIG. 10(A) o FIG. 10(A) shows a plan schematic view of the configuration of the light-receiving electrodes, wherein the pyroelectric member 41 is provided with two pairs of dual elements (i) and (ii) formed thereon. Thus, light-receiving electrodes 42, 43, 44, and 45 are connected, as shown in the electric circuit of FIG. 10(B). These light-receiving electrodes have a regular arcuate or arch shape about a common central point. These electrodes will output a signal on the plus side upon receiving infrared radiation on electrodes 42 and 44. Light-receiving electrodes 43 and 45 have a construction such that, upon receiving infrared radiation, the output will be on the minus side.
The light-receiving electrodes 42 and 43 have the same shape and the same area and are arranged to be symmetrical about a central point, and are also relatively close to each other to form a first dual element. The light-receiving elements 44 and 45 have both the same shape and light-receiving area, but are different from the light-receiving electrodes 42 and 43 in both shape and light-receiving area. Light-receiving electrodes 44 and 45 are likewise symmetrical about the same point and surround the light-receiving elements 42 and 43 to form the second dual element (ii). FIG. 10 discloses an electrical circuit schematic wherein high megohm load resistances 46 and 47 are provided, along with a pair of J-FET elements 48 and 49, which are capable of converting the input impedance. Supply voltage pins D.sub.i and D.sub.ii, along with output pins S.sub.i and S.sub.ii, are connected to the field effect transistors. Ground or earth pins are designated E.sub.i and E.sub.ii.
An output response of a pyroelectric detector 41 is shown in FIG. 11. A sensitivity orientation characteristic diagram is further shown in FIG. 12, and was obtained in the same manner as that disclosed in FIG. 9. As can be seen by making a comparison of these drawings, the dual twin-type pyroelectric detector has improved detection characteristics when compared with the dual-type pyroelectric infrared detector shown in FIG. 7.
Referring, however, to FIG. 11, the output of the dual electrodes is charted with regard to the moving direction shown by the vectors A, B, C, D, E, and F in FIG. 10(A). Thus, no output can be obtained in certain moving directions of the moving body, and sensitivity is greatly dependent upon the moving body direction. In particular, the sensitivity in the inclined direction shown by arrows E and F in FIG. 10 is deteriorated (see the chart of FIG. 12).
As shown in FIG. 11, the first dual electrode (i) is different from the second dual electrode (ii) in output form, since they are unbalanced sensing electrodes and they are not symmetric around the center of rotation to each other. Thus, the common mode noises due to disturbances, such as strong visible light, mechanical vibration, shock, etc., cannot be completely cancelled. This occurs because, for example, when external vibration and shock acts upon the first and second dual elements (i), (ii), the first dual element (i) is different from the second dual element (ii) in vibration characteristics. Thus, the pyroelectric output can be different in an absolute value between the first dual element (i) and the second dual element (ii). Therefore, they are not able to be directly offset. Additionally, if visible light is designed to be cut off, but a portion of a strong visible light comes through a multilayered interference filter (not shown) which has been provided for transmitting only infrared radiation from the target region, a difference in radiation can be produced between those dual elements (i) and (ii), which are asymmetric to each other.
The prior art has recently required expanding a sensing area to additionally include a position immediately below the pyroelectric infrared detector. These pyroelectric infrared detectors are being subject to utilization in environments having a large number of noise sources, such as various kinds of lighting, acoustic machines, tools, etc. Accordingly, there has been a desire to provide a pyroelectric infrared detector capable of eliminating the noises of a common mode due to visible rays, vibration, shock, and the like while addressing the problem of the dead angle detection regions that have existed in the prior art. Such a pyroelectric infrared detector, however, must still be capable of detecting the moving direction of a moving body in an economical manner.